基于宏微观分析的碳纤维增强高分子复合材料强度性能表征

微观力学强度理论(MMF)是一种新型的基于物理失效模式的复合材料强度理论。通过对碳纤维/树脂(UTS50/E51)复合材料单向层合板进行纵向、横向静载拉伸、压缩和弯曲试验, 得到层合板的基本力学性能和宏观强度指标。建立了碳纤维增强树脂基复合材料微观力学模型, 获取树脂基体和纤维不同位置的机械载荷应力放大系数和热载荷应力放大系数。结合获取的应力放大系数及试验测得的单向层合板宏观强度, 计算出层合板组分的MMF强度特征值。绘制了基于MMF强度理论的层合板破坏包络线, 并与Tsai-Wu失效准则预测结果进行对比。实现了对UTS50/E51层合板MMF强度特征值的表征。     

Simulation of inelastic response of multidirectional laminates based on stress failure criteria

Abstract

The primary purpose of this paper is to simulate the non-linear stress–strain curve of a multidirectional laminate subjected to an arbitrary in plane load using constituent material data and laminate geometrical parameters. The simulation is performed at a ply level. The classical laminated plate theory is employed to determine the load shared by each lamina in the laminate, while internal stresses in the constituent fibre and matrix of the lamina are obtained using a recently developed bridging micromechanics model. Thus, various failure criteria can be incorporated to detect the failure of a lamina in the laminate, and a progressive failure process is assumed by stiffness discount. Another objective of this paper is to investigate the influence of three typical failure criteria, i.e. the maximum normal stress criterion, the Tsai–Wu criterion, and the Hashin–Rotem criterion, on the simulation. Prediction has been made for T300/5208 graphite–epoxy laminates of a number of layups subjected to uniaxial tension. For the considered laminates, the predicted curves agree well with available experimental data. It is found that the predic tions based on the maximum normal stress criterion are comparable with those based on the other two criteria. As the maximum normal stress criterion is the simplest in application, it is recommended as the first candidate for laminate non-linear and failure analysis.     

An evaluation of failure criteria for matrix induced failure in composite materials

This paper reports on work being undertaken in the Cooperative Research Centre for Advanced Composite Structures Ltd. (CRC-ACS) to develop improved techniques for predicting the failure of composite materials. The procedures being investigated include a maximum strain criterion for fibre failure. For failure of the resin a new approach, which includes determination of the residual stresses due to manufacturing, is being trialed. This work closely parallels the new criteria proposed by Gosse and Hart-Smith [AIAA/CRC-ACS text on composite materials, submitted for publication] and we have subsequently replaced a simple stress criterion for matrix failure with their proposals based on strain invariants. The new procedures are applied to the failure of laminates in bolted joints with complex steered fibre patterns. Thermal residual stress was included to predict the matrix failure of T-section laminates under loads that open the angle between the flanges and the web. Here a transverse tension stress criterion was used.     

Optimization of laminated composites considering different failure criteria

The purpose of the present work is to analyse how different the optimal structures are when different first ply failure criterion are considered in the optimization of laminated composites. Two problems are solved: the minimum weight and the minimum material cost of laminated plates subjected to in-plane loads. The failure criterion is taken into account by means of constraints introduced in the optimization problem. Three different failure criteria are tested independently: maximum stress, Tsai–Wu and the Puck failure criterion (PFC). Emphasis is given to the PFC as it appears to agree better with practical observations. The design variables are the ply orientations, the number of layers and the layer material, and the optimization problem is solved by a genetic algorithm (GA). The results show that optimal structures highly differ when different failure criterion are considered and that none of the failure criteria is always the most or the least conservative when different load conditions are applied.     

Compressive behaviour of softwood under uniaxial loading at different orientations to the grain

The compressive behaviour of spruce wood under uniaxial loading is studied at different orientations with regard to the longitudinal and radial direction. The dependence of the Young modulus, Poisson ratio and crushing strength on the loading angle with respect to the longitudinal direction is shown and described by a simple theory of orthotropic elasticity and a Tsai–Hill like strength criterion. The deformation and failure behaviour was also strongly influenced by the loading orientation. The different deformation and failure mechanisms found ranged from buckling of the elongated cells at loading in the longitudinal direction followed by final failure due to longitudinal cracks to shear deformation and failure at annual ring borders at loading angles of 20° and 45° due to the abrupt density change at the ring border and to the plastic yielding and collapsing followed by densification for loading in the radial direction.     

Postbuckling response and failure of symmetric laminates under in-plane shear

This paper summarizes a study aimed at understanding the postbuckling behaviour and progressive failure of thin, simply supported symmetric rectangular laminates with various possible in-plane boundary conditions and under the action of in-plane shear loads. First-order shear deformation theory and geometric non-linearity, in the von-Karman sense, is used with a finite-element procedure. The 3D Tsai–Hill criterion is used to predict failure of lamina and the maximum stress criterion is used to predict the onset of delamination at the interface of two adjacent layers. The effect of in-plane boundary conditions, plate lay-ups, plate aspect ratio, fiber orientations and lamina material properties on the load deflection response, buckling load, first-ply failure load, ultimate load and the maximum transverse displacement associated with failure loads is presented.     

Compressive behaviour of thin-skin stiffened composite panels with a stress raiser

The in-plane compressive behaviour of thin-skin stiffened composite panels with a stress concentrator in the form of an open hole or low velocity impact damage is examined analytically. Drop weight impact in laminated polymer composites causes matrix cracking, delaminations and fibre breakage, which together can seriously degrade the laminate compressive strength. Experimental studies, using ultrasonic C-scan images and X-ray shadow radiography, indicated that the overall damage resembles a hole. Under uniaxial compression loading, 0° fibre microbuckling surrounded by delamination grows laterally (like a crack) from the impact site as the applied load is increased. These local buckled regions continued to propagate, first in discrete increments and then rapidly at failure load. The damage pattern is very similar to that observed in laminated plates with open holes loaded in compression. Because of this resemblance, a fracture mechanics model, developed initially to predict notched compressive strength, was applied to estimate the compression-after-impact (CAI) strength of a stiffened panel; in the analysis the impact damage is replaced with an equivalent open hole. Also, the maximum stress failure criterion is employed to estimate the residual compressive strength of the panel. The unnotched compressive strength of the composite laminate required in the analysis is obtained from a three-dimensional stability theory of deformable bodies. The influence of the stiffener on the compressive strength of the thin-skin panel is examined and included in the analysis. A good agreement between experimental measurements and predicted values for the critical failure load is obtained.     

A mechanically fastened composite laminate joint and progressive failure analysis

A comprehensive procedure for a mechanically fastened composite laminate joint (ASTM D5961 Proc. A, B) is demonstrated from fixture design to analysis of test results. The ASTM tests are applied to evaluate the standard laminate properties and the composite joints. Composite laminate mechanical joints were analyzed using the finite element method (FEM), and the results were compared to test results. A progressive failure analysis (PFA) was applied to the FEM to predict the overall failure behavior of the test specimens. Three laminate failure theories – maximum stress, maximum strain, and Tsai–Wu – were applied to the PFA to predict the test failure load, displacement and strength. The PFA method was suitable to predict the initial test range of test and maximum test load except for the excessive failure area.     

A comparative study of failure criteria in probabilistic fields and stochastic failure envelopes of composite materials

One of the major objectives of this paper is to offer a practical tool for materials design of unidirectional composite laminates under in-plane multiaxial load. Design-oriented failure criteria of composite materials are applied to construct the evaluation model of probabilistic safety based on the extended structural reliability theory. Typical failure criteria such as maximum stress, maximum strain and quadratic polynomial failure criteria are compared from the viewpoint of reliability-oriented materials design of composite materials. The new design diagram which shows the feasible region on in-plane strain space and corresponds to safety index or failure probability is also proposed. These stochastic failure envelope diagrams which are drawn in in-plane strain space enable one to evaluate the stochastic behavior of a composite laminate with any lamination angle under multi-axial stress or strain condition. Numerical analysis for a graphite/epoxy laminate of T300/5208 is shown for the comparative verification of failure criteria under the various combinations of multi-axial load conditions and lamination angles. The stochastic failure envelopes of T300/5208 were also described in in-plane strain space.     

Failure behavior of E-glass fiber- and fabric-reinforced IPC composites under tension and compression loading

This paper analyzes the failure behavior of E-glass reinforced inorganic phosphate cement composites in tension and compression. The first part of the study experimentally evaluates the behavior of different composite systems resulting from three different manufacturing processes (hand lay-up, pultrusion and pultrusion/compression). Tensile and compression tests are performed on fiber orientations ranging from 0° to 90°, and the mechanical tests are analyzed in terms of failure strength and failure modes. The prediction of failure strength is approached using mechanical models found in the literature. The comparison of experimental and modeling results is unsatisfactory, confirming the necessity of establishing adapted fracture criteria. The second part of this paper concerns the establishment of a failure envelope based on the Tsai–Wu and Mohr–Coulomb criteria. The anisotropic nature of the composite is accounted for by the Tsai–Wu criterion, while the Mohr–Coulomb criterion allows for the modeling of the mineral matrix’s brittle behavior. The maximum stress criterion is also compared to the obtained envelope curves.     

含冲击损伤复合材料层压板压缩破坏机制试验研究

为了研究含冲击损伤复合材料层压板的压缩破坏机制, 通过前后表面超声C扫描获得分层损伤在层压板内的分布情况, 使用应变片获取层压板两面的应变场, 采用声发射观察层压板压缩破坏过程。研究发现: 冲击分层损伤在厚度方向上的非对称分布引起层压板内部出现严重的应力集中, 致使纤维断裂并最终导致层压板整体破坏; 导致含冲击损伤层压板压缩破坏的主要原因为纤维断裂而非分层扩展。     

Matrix failure in composite laminates under compressive loading

The failure envelope of the matrix in composite laminates under compressive loads has not received much attention in literature. There are very little to no experimental results to show a suitable failure envelope for this constituent found in composites. With increasing popularity in the use of micromechanical analysis to predict progressive damage of composite structures which requires the use of individual failure criteria for the fibre and matrix, it is important that matrix behaviour under compression is modelled correctly.In this study, off-axis compression tests under uniaxial compression loading are used to promote matrix failure. Through the use of micromechanical analysis involving Representative Volume Elements, the authors were able to extract the principal stresses on the matrix at failure. The results indicated that hydrostatic stresses play an important role in the failure of the matrix. Thus, Drucker–Prager failure criterion is recommended when modelling compressive matrix failure in composite structures.     

Triple-scale failure estimation for a composite-reinforced structure based on integrated modeling approaches. Part 1: Microscopic scale analysis

The structural reliability of a composite component locally reinforced with a fibrous metal matrix composite is essentially affected by the micro-scale failures. The micro-scale failures such as fiber fracture or matrix damage are directly governed by the internal stress states such as mismatch thermal stress. A proper computational method is needed in order to obtain micro-scale stress data for arbitrary thermo-mechanical loads. In this work a computational scheme of microscale failure analysis is presented for a composite component. Micromechanics-based triple-scale FEM was developed using composite laminate element. The considered composite component was a plasma-facing component of fusion reactors consisting of a tungsten block and a composite cooling tube. The micro-scale stress and strain data were estimated for a fusion-relevant heat flux load. Ductile damage of the matrix was estimated by means of a damage indicator. It was shown that the risk of micro-scale composite failure was bounded below an acceptable level.     

Off-axis strength differential effects in unidirectional carbon/epoxy laminates at different strain rates and predictions of associated failure envelopes

A difference between off-axis tensile and compressive strengths in a unidirectional carbon/epoxy laminate is examined at 100 °C for different fiber orientations and strain rates. By comparing their predictions with experimental results, the Tsai–Wu, Hoffman, Hashin–Rotem failure criteria that can distinguish between the off-axis strengths in tension and compression are evaluated for the accuracy of prediction of the off-axis strength differential (SD) effect and of the failure envelopes associated with off-axis loading at different strain rates. It is shown that the failure envelope associated with off-axis compression is unsuccessfully predicted by these failure criteria. The comparison suggests that the SD effects in the longitudinal, transverse and shear strengths should be taken into account for accurate prediction of the off-axis failure envelope. On the basis of this experimental implication, simple modifications to the representative failure criteria are attempted in which both the normal and shear SD effects are taken into account.     

Progressive failure modeling of woven fabric composite materials using multicontinuum theory

Failure of composite materials often results from damage accumulation in the individual constituents (fiber and matrix) of the composite. At times, damage may even be limited to a single constituent. The ability to accurately predict not only ultimate strength values but also intermediate constituent level failures is crucial to the success of introducing composite materials into demanding structural applications.In this paper, we develop two progressive failure models for the analysis of a plain weave composite material. The formulations are based on treating the weave as consisting of separate but linked continua representing the warp fiber bundles, fill fiber bundles, and pure matrix pockets. Retaining constituent identities allows one to access constituent (phase averaged) stress fields that are used in conjunction with both a stress based and damage based failure criterion to construct a nonlinear progressive failure algorithm for the woven fabric composite material. The MCT decomposition and the nonlinear progressive failure algorithm are incorporated within the framework of a traditional finite element analysis.The constituent based progressive failure algorithm combined with both the stress based and damage based failure criteria are compared against experimental data for a plain weave, woven fabric composite under various loading conditions. The analytical results from the damage based approach show a marked improvement over the stress based predictions and are in excellent agreement with the experimental data.     

Fracture mechanisms and failure analysis of carbon fibre/toughened epoxy composites subjected to compressive loading

This study investigates the failure mechanisms of unidirectional (UD) HTS40/977-2 toughened resin composites subjected to longitudinal compressive loading. A possible sequence of failure initiation and propagation was proposed based on SEM and optical microscopy observations of failed specimens. The micrographs revealed that the misaligned fibres failed in two points upon reaching maximum micro-bending deformation and two planes of fracture were created to form a kink band. Therefore, fibre microbuckling and fibre kinking models were implemented to predict the compressive strength of UD HTS40/977-2 composite laminate. The analysis identified several parameters that were responsible for the microbuckling and kinking failure mechanisms. The effects of these parameters on the compressive strength of the UD HTS40/977-2 composite systems were discussed. The predicted compressive strength using a newly developed combined modes model showed a very good agreement to the measured value.     

Optimum in situ strength design of laminates under combined mechanical and thermal loads

In this paper, optimum laminate configurations are sought for multidirectional fibre-reinforced composite laminates under combined in-plane mechanical and thermal loads. The design objective is to enhance the value of the loads over and above the first-ply-failure loads which are judged by a transverse failure criterion and the Tsai-Hill criterion, respectively. The in situ strength parameters previously obtained are incorporated in these criteria. It is found that the optimum designs under combined mechanical and thermal loads are not the same as those under pure mechanical loads for three of the four loading cases studied. For all cases the optimum loads are significantly larger than those for a quasi-isotropic design.     

Stress Concentrations Caused by Embedded Optical Fiber Sensors in Composite Laminates

The fiber optic sensor (FOS) embedded perpendicular to reinforcing fibers causes an `Eye' shaped defect. The length is about 16 times fiber optic radius (R_Fos) and height is about 2R_Fos. The eye contains fiber optics in the center surrounded by an elongated resin pocket. Embedding FOS causes geometric distortion of the reinforcing fiber over a height equal to 6 to 8 R_Fos. This defect causes severe stress concentration at the root of the resin pocket, the interface (in the composite) between the optical fiber and the composite, and at 90° to load direction in the composite. The stress concentration was calculated by finite element modeling of a representative micrograph. The FE results agreed reasonably with analytical and experimental data in the literature for a similar problem. The stress concentration in axial direction was about 1.44 and in transverse direction at the interface was -0.165 and at resin pocket was 0.171. Under tensile loading, the initial failure was by transverse matrix cracking (fiber splitting) at the root of the resin pocket, then that lead to final fracture by fiber breakage. Under compression loading, the failure initiation was by interfacial cracking due to large transverse tensile stress and the final fracture was by compression. Fracture stress calculated from the analysis using the maximum stress criteria agreed reasonably with test data.     

Interfiber/interlaminar failure of composites under multi-axial states of stress

Unidirectional and textile carbon/epoxy composites were characterized under multi-axial states of stress. In-plane and through-thickness tensile, compressive, and shear tests were conducted at various orientations with the principal material axes. The stress–strain behavior, failure modes, and strengths were recorded. Results were compared with three types of failure criteria in three dimensions, limit criteria (maximum stress), fully interactive criteria (Tsai-Hill, Tsai-Wu), and failure mode based and partially interactive criteria (Hashin–Rotem, Sun, NU). The latter, a new interfiber/interlaminar failure theory developed by the authors, was found to be in excellent agreement with experimental results, especially in cases involving interfiber/interlaminar shear and compression. Of special note was the failure mode in transverse compression, where the failure plane was not predictable by conventional composite failure theories. The orientation of the failure plane was more in line with predictions by a Mohr–Coulomb failure model.     

基于三维Puck失效准则及唯象模量退化的复合材料臂杆屈曲分析

针对碳纤维增强树脂基复合材料（CFRP）臂杆结构在压缩和扭转载荷条件下屈曲与后屈曲问题,采用三维Puck失效准则和基于唯象分析的模量退化方法,同时考虑层合结构就位效应及沿纤维方向应力对横向强度的影响,建立了一种适用于考虑渐进失效CFRP结构的屈曲分析方法,并通过编写有限元软件ANSYS的USERMAT子程序进行了数值实现。与文献中实验结果的对比表明,上述方法能够分析复合材料结构的渐进失效过程和后屈曲承载特性,预测精度高。进而采用此方法,详细分析了某航天器臂杆结构在承受压缩与扭转载荷条件下的屈曲载荷及后屈曲特性。